This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
A common practice in the processing of seismic data is to remove the source and receiver signatures from the seismic data. For marine seismic data acquisition, the removal of the source signature requires accurate estimation of the notional signatures at each source location in an array of sources. Since Zilkowski U.S. Pat. No. 4,476,550, there has been a continuous effort to create a shot-by-shot estimate of the far-field signature of an air gun source array. Brac U.S. Pat. No. 4,827,456 selected from a catalog of notional signatures based on key parameters measured at the time of acquisition. Newman U.S. Pat. No. 4,693,336 fired a point source with a known far-field signature just before the primary source array was fired and then computed a transfer function. Parkes U.S. Pat. No. 7,218,572 applied calibration functions and measured physical parameters to the air gun signature model. Hoogeveen U.S. Pat. No. 7,539,079 used separate seismic transmitters or the air guns to locate the air gun sources via travel times. Hopperstad U.S. Pat. No. 7,440,357 proposed to include sea-surface and seafloor reflections of signals in Zilkowski's U.S. Pat. No. 4,476,550 formulation to compute notional signatures. Yang U.S. Pat. No. 8,917,573 used GPS to measure the source array geometry. Laws US20090073804 compensated for height and shape of the sea surface. Hopperstad U.S. Pat. No. 8,605,551 corrected the source array geometry using the near-field hydrophones in the active source array. Hopperstad U.S. Pat. No. 8,687,462 quantified differences between two identical sources within the active source array. Laws US20130279291 computed the notional source signature utilizing two near-field hydrophones at each source location. Hegna US20150234071 redefined the traditional definition of a notional source and computed a transfer function between two locations and then used the transfer function to predict a third point. For seismic acquisition in shallow water, all of these methodologies' ability to compute an accurate far-field estimate is limited by the presence of the ocean bottom and subsurface reflections and by the shot-to-shot variations in the source array geometry. Consequently, estimating and removing surface and subsurface reflection energy from near-field hydrophone data is a problem that conventional techniques have not solved with respect to estimating a source array's far-field signature.
It is common practice in the seismic industry to utilize two air gun source arrays to acquire the data for a 3D or 4D seismic survey. This mode of operation is referred to as “flip-flop” shooting. In this shooting mode, the two source arrays are side-by-side with a crossline spacing of approximately 25 m between the physical centers of the two source arrays. This acquisition arrangement is shown in FIG. 1, which illustrates a plan view of two successive shots in a marine seismic survey using two air gun arrays in a flip-flop style shooting mode. There are 18 gun stations 101 in each source array, with a near-field hydrophone (NFH) disposed above each gun station. Gun stations 102 are two sources that are close enough together (less than 1 m apart) to be treated as a single gun station. In this FIG. 1, two successive shots are shown with the starboard array firing on shot N and the port array firing on shot N+1 (i.e. the flip and the flop of flip-flop). Seismic acquisition vessels can be configured with more than two air gun arrays, but this is less common.
A common practice, as described in Mott-Smith U.S. Pat. No. 3,893,539, has been to place a hydrophone close to each gun station in an air gun array. This practice is illustrated in FIG. 1 by the small gray circles located at each gun station in the port and starboard source arrays. The hydrophone at each gun station has come to be referred to as a near-field hydrophone, NFH, because these hydrophones are typically placed approximately one meter above the air gun(s) that are located at each gun station. This close proximity to an air gun source causes the hydrophone to be within the near-field of the air gun(s) the hydrophone is located above. The NFH is typically a dynamic pressure sensor used to measure pressure and/or particle motion or to measure analogs of pressure and/or particle motion. The term near-field hydrophone is a simplification in regards to what the sensor is capable of measuring, as each sensor can measure a myriad of signals when any or all of the air guns are fired.
Near-field hydrophones have been primarily used as quality control devices for the individual air guns in an air gun array (Mott-Smith U.S. Pat. No. 3,893,539) and as a quality control for the performance and variability of an air gun array (Parkes 1984, Ziolkowski 1997, Brink 1999, Hegna US20080175102). Ziolkowski in U.S. Pat. No. 4,476,550 describes that “notional” air gun signatures can be determined from near-field hydrophones placed at each air gun station and that the notional signatures can be used to generate the far-field signature of the air gun array. In this context, a notional source signature is the pressure field generated at an air gun station without interaction with any other air gun sources and without any energy reflected from the ocean surface, the ocean bottom or the ocean subsurface. Ziolkowski's method uses nominal geometry for the air gun locations and assumes that the air guns within a source array do not interact. In practice, these assumptions break down because typical air gun towing arrangements allow the spacing between the air gun strings to vary along a seismic line. This variation is most evident in the bubble portion of the near-field hydrophone data and is the basis of one technique used to identify changes in the air gun string geometry and changes in the air guns used in a source array.
Near-field hydrophones used in source arrays differ from the hydrophones located in the streamer cables. Table 1 compares the characteristics of streamer hydrophones and near-field hydrophones. There clearly are differences but both sensors are designed to measure dynamic changes in pressure with a significant level of fidelity over a significant range of amplitudes. The principle difference between these sensors is that the near-field hydrophones are designed to survive at pressures well in excess of one bar where streamer hydrophones are designed to measure reflection signal amplitudes on the order of a few microbar. Since both the streamer and near-field hydrophones have dynamic ranges of 90+dB, their recording ranges overlap for a significant portion of amplitude range exhibited by an air gun shot.
One very important additional difference between streamer hydrophones and near-field hydrophones is the minimum offset that is recorded with the sensors. The near-field hydrophones embedded in the active source array can record data that is substantially zero offset data. In this context, substantial means the data recorded by the sensor is well within the coherency distance associated with near surface reflection events and within a single binning cell used to process the seismic data. The near-field hydrophones in the inactive array acquire near-zero offset data, typically 25 m offset data. The minimum offset the streamer hydrophones can record is on the order of 90 to 100+m because of the configuration used to tow the streamers and the air gun arrays. This source to receiver distance results in data or traces missing from the acquisition for the small offsets between source and nearest receivers in the acquired seismic data records. Near offset data refers to data that would have been recorded by a receiver that is closer to the source than any of the survey receivers on the streamers. In water depths greater than 200 m, the minimum streamer offset of 90 to 100+m poses some processing issues but the problems can be surmounted. In water depths less than 200 m, the minimum streamer offset causes important pre-critical water bottom reflection information to not be recorded on the streamer hydrophones. This missing data can seriously impact the quality and accuracy of the seismic image.
TABLE 1Comparison of typical near-field hydrophones to streamer hydrophonecharacteristicsNear-fieldStreamerCharacteristicHydrophoneHydrophoneConstructionDesigned for largeDesigned foramplitude signalssmallin excess of 5 baramplitudesignals as smallas microbarsSensitivity (V/bar) 7 to 1020Capacitance (nF) 8 to 12278Typical digitizers24-bit sigma delta24-bit sigmadeltaSampling interval0.1 to 0.52(ms)Dynamic range90 to 9595 to 105(dB)AccelerationNoYescancelingNumber ofOne per gun1 to 8 perhydrophones perstationgroupdata channelSpatial dimensionSingle element12.6(m)AccelerationNoYescancelingMinimum offsetA few meters100 130(m)
Since the air gun source array with the embedded NFH is being towed through the water, the location of the NFH is somewhat spatially displaced with respect to the location where the air guns were fired. Typically, air gun arrays are towed through the water at 5 knots (2.5722 m/s). For nominal water velocities, a water depth of 100 m and a towing speed of 5 knots, the NFH would be displaced 0.34 m from the zero offset location. Although this distance is non-zero, the term zero-offset will be used because seismic data measurements at this very near zero offset distance are significant improvement over the 90 m to 150 m offsets that are available from typical production seismic geometries.
The air gun is the most commonly used source generator for marine seismic acquisition associated with petroleum exploration. In operation, the air gun is pressurized (typical gun pressures range from 2000-2500 psi) which stores compressed air in a high pressure chamber. Typically the volume of the high pressure chamber ranges from 20 cubic inches to 350 cubic inches. After the air chamber is fully pressurized, the air gun is fired and releases the compressed air into the water creating a primary pulse which has a typical duration of 10 to 12 ms. Following the primary pulse, a series of additional bubble pulses are created by the expansion-collapse cycles of the air bubble as it rises to the sea surface (Dragoset, 2000). Depending on the depth at which the air gun is fired, the total air gun signature, which includes the primary impulse and the bubble pulse train, can have a duration of 500 to 1000+ms.
The time duration to the peak amplitude of the first bubble pulse is principally a function of the volume of the high pressure chamber, the operating pressure, the depth of the air gun and the type of air gun. This time duration increases as the volume of the high pressure and the operating pressure are increased and it decreases as the depth of the air gun is increased. Additionally, when multiple air guns are fired simultaneously, the character of the bubble pulse train generated by an individual air gun can be altered by its proximity to the other air guns. The pressure source response is further complicated by the mirror reflection from the ocean's surface of a pressure wave generated by a marine energy source. Mathematically the ocean surface reflection is accommodated by including a so-called source ghost at the same distance above the ocean surface as the true source is below the ocean surface. The pressure wave field ghost response is polarity reversed compared to the true air gun source response and has a magnitude proportional to sea surface reflectivity. The sea surface reflectivity magnitude appears to be a function of frequency with an amplitude range from unity at zero frequency to something with smaller magnitude at higher frequencies. The sea state and the reflection angle alter the magnitude of the mirrored pressure response. High frequencies are scattered by a rough sea surface more than low frequencies effectively lowering the magnitude of their comparative sea surface reflectivity.
The interpretability of seismic data can be improved by improving the ratio of the amplitude of the primary pulse to the amplitude of the initial bubble pulse for a seismic source. This ratio is called the peak-to-bubble ratio. Increasing the peak-to-bubble ratio and the need to have detectable reflection energy from a petroleum reservoir motivates the use of multiple individual air guns in the air gun arrays used for marine seismic acquisition. A single air gun with a specified gun volume typically has a fairly low primary-to-bubble ratio. An air gun array with all guns of the same size would marginally improve the primary-to-bubble ratio. Arranging air guns with different volumes in an array and aligning the primary pulses for the individual air guns in the array causes the primary pulses to constructively sum while simultaneously the bubble pulses from different gun volumes to destructively sum. Various designs of air gun arrays are often introduced to meet other practical purposes as well, e.g., alter the bandwidth coverage. Usually a modern air gun array consists of two to four strings of air guns with about 8 to 10 meters distance between the strings and having 10 to 16 air guns or air gun clusters mounted at 6 to 8 gun stations on each string.
FIG. 2 displays near-field hydrophone traces for two successive air gun shots acquired in a flip-flop fashion, wherein a constant gain was used for all traces. The primary pulse 201 and the bubble pulse train 202 are clearly evident on the near-field hydrophones associated with the source array that is being fired, i.e. the active array. The mirrored reflection from the sea surface 203 is difficult to identify because of its reduced amplitude due to 1/r amplitude losses associated with its travel from the air gun to the surface and from the surface back to the near-field hydrophone. The direct arrival 201 and the mirrored surface reflection 203 are much easier to see in the near-field hydrophones associated with the passive source array, i.e. the inactive array. The direct arrivals include signals which travel directly from their point of origin to the recording receivers without having undergone a reflection or refraction. Additionally, the water bottom reflection 203 can be seen on the near-field hydrophones in the inactive array due to the 1/r traveltime, amplitude reduction of the direct arrivals and the mirrored surface reflection.
Applying a 200-ms AGC gain to the near-field hydrophone traces shown in FIG. 2 and plotting a density display under the plotted traces, FIG. 3, allows the water bottom reflection 301, multiples of the water bottom reflection 302, subsurface reflections 303, and direct arrivals 304 to be easily recognized. These same reflection events are also present on the near-field hydrophones in the active source array, but the events are obscured by the air gun signature (i.e. the initial pressure pulse and the bubble train plus the surface mirroring). These near-field hydrophone traces were acquired in a water depth of approximately 130 m. As the water depth decreases, the events become more overlapped and increasingly difficult to identify.
Clearly the direct arrival portion of the near-field trace could be attenuated with a mute as suggested by Kragh U.S. Pat. No. 8,958,266, but doing so would discard a great deal of the near-surface reflection events with the consequence that some shallow multiple generators will not be identified. An alternate approach is suggested by analyzing the spectral content of an air gun far-field signature as a function of time, i.e. a spectrogram. Such an analysis is shown in FIG. 5, wherein the air gun signature that was analyzed in on the right side of the display and a plot of the percent of the total energy as a function of time is plotted on the left side of the display. From this analysis, after 250 to 300 ms, the air gun energy lies below 40 Hz. Applying a 40 Hz low-cut filter would allow some of the reflection events to be identified, but very near-surface events would still be obscured and a significant portion of the usable seismic bandwidth would have been discarded. Removing frequencies below 40 Hz would be especially detrimental to full wavefield inversion (FWI) processing where the low frequency content is so important.
The problem of obscuring reflection events is significantly reduced for the streamer hydrophones because their minimum offset from the source arrays is large and the added distance attenuates the amplitude of the ocean bottom and subsurface reflections. FIG. 4 is a typical common shot gather of streamer hydrophone traces that are nearest to the source array. The water depth of approximately 130 m separates the direct arrivals 401 from the ocean bottom reflection 402 and the subsurface reflections 403. Unfortunately as the water depth decreases, these events will start to overlap.
FIG. 6 illustrates a common shot, trace display of the near-field hydrophones for string 2 gun station 2 and string 5 gun station 2. These two stations are in the same relative position in source array 1 and source array 2. The trace amplitudes are in units of bar. As can be seen in FIG. 6, the air gun signature 603 duration is on the order of 500 to 1000 ms long. Even when the initial air gun signature can be visually identified, the energy associated with the ocean bottom 601 and subsurface reflection events 602 overlap and interfere. The degree of overlap and interference increases as the water depth decreases.
Converting the trace amplitudes shown FIG. 6 to dB values allows the amplitude disparities to be more easily quantified. In FIG. 7, the direct arrivals 701 for the near-field hydrophones in the active array are ˜18 dB larger than the direct arrivals observed at the near-field hydrophones in the inactive array. The water bottom reflection 702 is ˜33 dB smaller than the direct arrival at the active array and ˜15 dB smaller than the direct arrival at the inactive array. The subsurface reflections 703 are more than 24 dB smaller than the direct arrival at the inactive array and more than 42 dB smaller than the direct arrival at the active array. Earlier it was noted that the signature of an air gun is 500 to 1000+ms in duration. This observation is verified in FIG. 7 where the amplitude of the active array trace does not drop below 40 dB until sometime after 1000 ms.
Norris U.S. Pat. No. 8,964,502 provides a methodology to overcome these large amplitude differences, but it requires the calculation of a statistical estimation of the direct arrivals and assumes that the shot-to-shot variations in the air gun arrays are not significant. Depending on the sea state and the condition of the air gun arrays, this is not always a valid assumption. Additionally as noted in U.S. Pat. No. 8,964,502, the statistical estimation must be recomputed when air guns are added to or dropped from the air gun arrays. This method does not mute the direct arrivals so near-surface reflectors are preserved and it does not use frequency domain filters so the near-field hydrophone data and the streamer hydrophone data share a common frequency bandwidth. As noted in U.S. Pat. No. 8,964,502, once the direct arrival energy is removed, the near-field hydrophone traces for a shot can be summed or manipulated to improve the signal-to-noise ratio of the zero offset and near-zero offset near-field hydrophone traces. Removing the direct arrivals from the near-field hydrophone traces, provides an array of subsurface, single sensor data traces with varying offsets which are clustered around the zero offset location of the active source array and around the near-zero offset point located at the center of the inactive source array. It would be valuable to retain these positive characteristics provided by U.S. Pat. No. 8,964,502 while computing the direct arrival estimates on a shot-by-shot basis. Doing so could incorporate shot-to-shot variations in the estimation of the direct arrivals and would automatically accommodate shot-to-shot loss or addition of air guns.
Ideally, the streamer hydrophones would be deployed so the first streamer hydrophone had a zero offset with respect to the center of the source array. Such a deployment is not possible, nor practical, even in deep water because of the towing arrangement that is currently used and the sensitivity and fragility of the streamer hydrophones. Shallow water only exacerbates these issues.